The present invention relates to the removal of selenium from water, and more particularly but not exclusively to the treatment of drainage water containing selenium leached from soil.
Selenium is often present in water at concentrations of up to 0.5 mg/l, but a concentration of this order is undesirable for purposes such as drinking water or crop irrigation. For example, in the USA, the permitted maximum selenium level for drinking water is set at 10 .mu.g/l. In some selenium-containing waters such as certain irrigation drainage waters, a substantial proportion of the selenium is present as the selenate anion, [SeO.sub.4 ].sup.-2. These waters are especially difficult to treat because there is also a very high concentration of sulphate.
In the USA, particular attention has focussed on treatment of the agricultural drainage water in the San Joaquin Valley. This Valley has about 1.1 million acres of extremely productive land which is under irrigation. The land is generally low-lying and requires drainage in order to avoid high salinity in crop root zones. A major scheme to drain off subsurface water after use for irrigation has been frustrated by discovery of high levels of contaminants. The drainage water is brackish, and has a relatively high concentration of salts and potentially toxic elements, with selenium representing a particular problem. Environmental concern has led to a suspension of construction work, a closure order on a large reservoir, and an extensive program of drain plugging. With the plugging of the drains, the water table is beginning to rise, causing concern for growers in the Valley.
Methods are available for the purification of water, but in general the known methods are not suited for use on a large scale with brackish drainage water containing selenium and a range of other contaminants. Reverse osmosis is costly, and so also is ion exchange. In this respect, it is to be borne in mind that the initial aim in treating the drainage water in the San Joaquin Valley and elsewhere is to render it suited for discharge. Hence, an economic process is paramount.
Methods are also described in the literature which are specifically concerned with the removal of selenium from water, but it is not apparent that such methods can be used economically on a large scale.
U.S. Pat. No. 4,405,464 describes a process for the removal of selenium by a chemical treatment involving reduction of Se(VI) to Se(IV) using metallic iron which is itself oxidized and forms ferric hydroxide which then entrains the reduced selenium.
An article by Smith and Wiechers in Water SA (1981) 7, 65 describes elimination of toxic metals from waste water by an integrated wastewater treatment/water reclamation system. The system relies upon a combination of biological and chemical processes, including a bacteriological denitrification, clarification through formation of ferric hydroxide, and an anaerobic digestion.
An article by Gersberg and Elkins in Proceedings of Symposium on Selenium in the Environment, Cal. State University, Fresno, June 10-12, 1985, describes selenium removal using an immobilized cell reactor containing Pseudomonas species immobilized in alginate. A selenium medium containing 1975 mg/l of selenium was recirculated through the reactor to give a final concentration of 1180 mg/l selenium. In further experiments with the Pseudomonas, removal efficiencies of 44 and 68% were achieved.